Method and apparatus for image encoding and decoding using intra prediction

ABSTRACT

Provided are a method and apparatus for intra predicting an image, which generate a prediction value via linear interpolation in horizontal and vertical directions of a current prediction unit. The method includes: generating first and second virtual pixels by using at least one adjacent pixel located upper right and lower left to a current prediction unit; obtaining a first prediction value of a current pixel via linear interpolation using an adjacent left pixel located on the same line as the first virtual pixel and the current pixel; obtaining a second prediction value of the current pixel via linear interpolation using an adjacent upper pixel located on the same column as the second virtual pixel and the current pixel; and obtaining a prediction value of the current pixel by using the first and second prediction values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation application of U.S. application Ser. No.14/130,095 filed Jan. 31, 2014, which is a National Stage applicationunder 35 U.S.C. § 371 of PCT/KR2012/005148, filed on Jun. 28, 2012,which claims the benefit of U.S. Provisional Application No. 61/501,969,filed on Jun. 28, 2011, all the disclosures of which are incorporatedherein in their entireties by reference.

BACKGROUND

1. Field

Apparatuses and methods consistent with exemplary embodiments relate toencoding and decoding of an image, and more particularly, to intraprediction encoding and intra prediction decoding of an image, whereincompression efficiency of an image is improved by using variousdirectivities and a new intra prediction mode.

2. Description of the Related Art

In an image compression method, such as Moving Picture Experts Group(MPEG)-1, MPEG-2, MPEG-4, or H.264/MPEG-4 Advanced Video Coding (AVC), apicture is divided into macroblocks in order to encode an image. Each ofthe macroblocks is encoded in all encoding modes that can be used ininter prediction or intra prediction, and then is encoded in an encodingmode that is selected according to a bit rate used to encode themacroblock and a distortion degree of a decoded macroblock based on theoriginal macroblock.

As hardware for reproducing and storing high resolution or high qualityvideo content is being developed and supplied, there is an increasingneed for a video codec capable of effectively encoding or decoding thehigh resolution or high quality video content. In a related art videocodec, a video is encoded in units of macroblocks each having apredetermined size.

SUMMARY

Aspects of one or more exemplary embodiments provide methods andapparatuses for intra prediction encoding and intra prediction decodingof an image, wherein coding efficiency is improved according to an imagecharacteristic via a new intra prediction method using pixels adjacentto a current prediction unit.

Aspects of one or more exemplary embodiments also provide a new intraprediction mode using pixels adjacent to a current prediction unit.

According to aspects of one or more exemplary embodiments, encodingefficiency of an image can be improved by applying an optimum intraprediction method according to image characteristics via various intraprediction methods using adjacent pixels.

According to an aspect of an exemplary embodiment, there is provided amethod of intra predicting an image, the method including: obtaining afirst virtual pixel located on a same line as a current predicted pixelof a current prediction unit while corresponding to a pixel locatedfarthest right of the current prediction unit, by using at least oneadjacent pixel located upper right to the current prediction unit;obtaining a second virtual pixel located on a same column as the currentpredicted pixel while corresponding to a pixel located farthest belowthe current prediction unit, by using at least one adjacent pixellocated lower left to the current prediction unit; obtaining a firstprediction value of the current predicted pixel via linear interpolationusing the first virtual pixel and an adjacent left pixel on the sameline as the current predicted pixel; obtaining a second prediction valueof the current predicted pixel via linear interpolation using the secondvirtual pixel and an adjacent upper pixel on the same column as thecurrent predicted pixel; and obtaining a prediction value of the currentpredicted pixel by using the first and second prediction values.

According to an aspect of another exemplary embodiment, there isprovided an apparatus for intra predicting an image, the apparatusincluding: an intra predictor configured to obtain a first virtual pixellocated on a same line as a current predicted pixel of a currentprediction unit while corresponding to a pixel located farthest right ofthe current prediction unit, by using at least one adjacent pixellocated upper right to the current prediction unit, to obtain a secondvirtual pixel located on a same column as the current predicted pixelwhile corresponding to a pixel located farthest below the currentprediction unit, by using at least one adjacent pixel located lower leftto the current prediction unit, to obtain a first prediction value ofthe current predicted pixel via linear interpolation using the firstvirtual pixel and an adjacent left pixel on the same line as the currentpredicted pixel, to obtain a second prediction value of the currentpredicted pixel via linear interpolation using the second virtual pixeland an adjacent upper pixel on the same column as the current predictedpixel, and to obtain a prediction value of the current predicted pixelby using the first and second prediction values.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an apparatus for encoding a video,according to an exemplary embodiment;

FIG. 2 is a block diagram of an apparatus for decoding a video,according to an exemplary embodiment;

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment;

FIG. 4 is a block diagram of an image encoder based on coding unitsaccording to an exemplary embodiment;

FIG. 5 is a block diagram of an image decoder based on coding unitsaccording to an exemplary embodiment;

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions according to an exemplary embodiment;

FIG. 7 is a diagram for describing a relationship between a coding unitand transformation units, according to an exemplary embodiment;

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment;

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment;

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units, prediction units, and transformation units, according toan exemplary embodiment;

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transformation unit,according to encoding mode information of Table 1;

FIG. 14 is a table showing a number of intra prediction modes accordingto a size of a prediction unit, according to an exemplary embodiment;

FIG. 15 is a reference diagram for describing intra prediction modeshaving various directivities, according to an exemplary embodiment;

FIG. 16 is a diagram for describing a relationship between a currentpixel and adjacent pixels disposed on an extension line having adirectivity of (dx, dy), according to an exemplary embodiment;

FIGS. 17 and 18 are diagrams showing directions of an intra predictionmode, according to exemplary embodiments;

FIG. 19 is a diagram showing directions of an intra prediction modehaving 33 directivities, according to an exemplary embodiment;

FIGS. 20A and 20B are diagrams for describing a planar mode according toexemplary embodiments;

FIG. 21 is a diagram showing adjacent pixels that are filtered around acurrent prediction unit, according to an exemplary embodiment;

FIG. 22 is a reference diagram for describing a filtering process of anadjacent pixel; and

FIG. 23 is a flowchart illustrating an intra prediction method accordingto a planar mode, according to an exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments will be described more fully withreference to the accompanying drawings, in which like reference numeralsrefer to like elements throughout.

FIG. 1 is a block diagram of a video encoding apparatus 100, accordingto an exemplary embodiment.

The video encoding apparatus 100 includes a maximum coding unit splitter110, a coding unit determiner 120, and an output unit 130 (e.g.,outputter).

The maximum coding unit splitter 110 may split a current picture basedon a maximum coding unit for the current picture of an image. If thecurrent picture is larger than the maximum coding unit, image data ofthe current picture may be split into the at least one maximum codingunit. The maximum coding unit according to an exemplary embodiment maybe a data unit having a size of 32×32, 64×64, 128×128, 256×256, etc.,wherein a shape of the data unit is a square having a width and lengthin squares of 2. The image data may be output to the coding unitdeterminer 120 according to the at least one maximum coding unit.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes a number of times thecoding unit is spatially split from the maximum coding unit, and as thedepth deepens, deeper encoding units according to depths may be splitfrom the maximum coding unit to a minimum coding unit. A depth of themaximum coding unit is an uppermost depth and a depth of the minimumcoding unit is a lowermost depth. Since a size of a coding unitcorresponding to each depth decreases as the depth of the maximum codingunit deepens, a coding unit corresponding to an upper depth may includea plurality of coding units corresponding to lower depths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an exemplary embodiment is split according to depths, theimage data of a spatial domain included in the maximum coding unit maybe hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split may be predetermined.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths, and determines a depth to output a finally encoded image dataaccording to the at least one split region. In other words, the codingunit determiner 120 determines a coded depth by encoding the image datain the deeper coding units according to depths, according to the maximumcoding unit of the current picture, and selecting a depth having theleast encoding error. The determined coded depth and the encoded imagedata according to the determined coded depth are output to the outputunit 130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or below themaximum depth, and results of encoding the image data are compared basedon each of the deeper coding units. A depth having the least encodingerror may be selected after comparing encoding errors of the deepercoding units. At least one coded depth may be selected for each maximumcoding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units correspond to same depth inone maximum coding unit, it is determined whether to split each of thecoding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of the each coding unit,separately. Accordingly, even when image data is included in one maximumcoding unit, the encoding errors may differ according to regions in theone maximum coding unit, and thus the coded depths may differ accordingto regions in the image data. Thus, one or more coded depths may bedetermined in one maximum coding unit, and the image data of the maximumcoding unit may be divided according to coding units of at least onecoded depth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in the maximum coding unit. The ‘codingunits having a tree structure’ according to an exemplary embodimentinclude coding units corresponding to a depth determined to be the codeddepth, from among all deeper coding units included in the maximum codingunit. A coding unit of a coded depth may be hierarchically determinedaccording to depths in the same region of the maximum coding unit, andmay be independently determined in different regions. Similarly, a codeddepth in a current region may be independently determined from a codeddepth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto the number of splitting times from a maximum coding unit to a minimumcoding unit. A first maximum depth according to an exemplary embodimentmay denote the total number of splitting times from the maximum codingunit to the minimum coding unit. A second maximum depth according to anexemplary embodiment may denote the total number of depth levels fromthe maximum coding unit to the minimum coding unit. For example, when adepth of the maximum coding unit is 0, a depth of a coding unit, inwhich the maximum coding unit is split once, may be set to 1, and adepth of a coding unit, in which the maximum coding unit is split twice,may be set to 2. Here, if the minimum coding unit is a coding unit inwhich the maximum coding unit is split four times, 5 depth levels ofdepths 0, 1, 2, 3 and 4 exist, and thus the first maximum depth may beset to 4, and the second maximum depth may be set to 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, according to the maximumcoding unit.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For convenience ofdescription, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in a maximum codingunit.

The video encoding apparatus 100 may variously select a size or shape ofa data unit for encoding the image data. In order to encode the imagedata, operations, such as prediction encoding, transformation, andentropy encoding, are performed, and at this time, the same data unitmay be used for all operations or different data units may be used foreach operation.

For example, the video encoding apparatus 100 may select not only acoding unit for encoding the image data, but also a data unit differentfrom the coding unit so as to perform the prediction encoding on theimage data in the coding unit.

In order to perform prediction encoding in the maximum coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split to coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will now be referred to as a ‘predictionunit’. A partition obtained by splitting the prediction unit may includea prediction unit or a data unit obtained by splitting at least one of aheight and a width of the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, and a size ofa partition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partitiontype include symmetrical partitions that are obtained by symmetricallysplitting a height or width of the prediction unit, partitions obtainedby asymmetrically splitting the height or width of the prediction unit,such as 1:n or n:1, partitions that are obtained by geometricallysplitting the prediction unit, and partitions having arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, a inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having a leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based not only on the coding unit forencoding the image data, but also based on a data unit that is differentfrom the coding unit.

In order to perform the transformation in the coding unit, thetransformation may be performed based on a data unit having a sizesmaller than or equal to the coding unit. For example, the data unit forthe transformation may include a data unit for an intra mode and a dataunit for an inter mode.

A data unit used as a base of the transformation will now be referred toas a ‘transformation unit’. Similarly to the coding unit, thetransformation unit in the coding unit may be recursively split intosmaller sized regions, so that the transformation unit may be determinedindependently in units of regions. Thus, residual data in the codingunit may be divided according to the transformation unit having the treestructure according to transformation depths.

A transformation depth indicating the number of splitting times to reachthe transformation unit by splitting the height and width of the codingunit may also be set in the transformation unit. For example, in acurrent coding unit of 2N×2N, a transformation depth may be 0 when thesize of a transformation unit is 2N×2N, may be 1 when the size of thetransformation unit is thus N×N, and may be 2 when the size of thetransformation unit is thus N/2×N/2. In other words, the transformationunit having the tree structure may be set according to thetransformation depths.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth, but alsoabout information related to prediction encoding and transformation.Accordingly, the coding unit determiner 120 not only determines a codeddepth having a least encoding error, but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transformation unit for transformation.

Coding units according to a tree structure in a maximum coding unit anda method of determining a partition, according to exemplary embodiments,will be described in detail later with reference to FIGS. 3 through 12.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The output unit 130 outputs the image data of the maximum coding unit,which is encoded based on the at least one coded depth determined by thecoding unit determiner 120, and information about the encoding modeaccording to the coded depth, in bitstreams.

The encoded image data may be obtained by encoding residual data of animage.

The information about the encoding mode according to coded depth mayinclude information about the coded depth, about the partition type inthe prediction unit, the prediction mode, and the size of thetransformation unit.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output, and thusthe split information may be defined not to split the current codingunit to a lower depth. Alternatively, if the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth, and thus the split information maybe defined to split the current coding unit to obtain the coding unitsof the lower depth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is repeatedly performed on eachcoding unit of the lower depth, and thus the encoding may be recursivelyperformed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit according to an exemplary embodiment is a square dataunit obtained by splitting the minimum coding unit constituting thelowermost depth by 4. Alternatively, the minimum unit may be a maximumsquare data unit that may be included in all of the coding units,prediction units, partition units, and transformation units included inthe maximum coding unit.

For example, the encoding information output through the output unit 130may be classified into encoding information according to coding units,and encoding information according to prediction units. The encodinginformation according to the coding units may include the informationabout the prediction mode and about the size of the partitions. Theencoding information according to the prediction units may includeinformation about an estimated direction of an inter mode, about areference image index of the inter mode, about a motion vector, about achroma component of an intra mode, and about an interpolation method ofthe intra mode. Also, information about a maximum size of the codingunit defined according to pictures, slices, or GOPs, and informationabout a maximum depth may be inserted into a header of a bitstream.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing a height or width of a coding unit ofan upper depth, which is one layer above, by two. In other words, whenthe size of the coding unit of the current depth is 2N×2N, the size ofthe coding unit of the lower depth is N×N. Also, the coding unit of thecurrent depth having the size of 2N×2N may include maximum 4 of thecoding unit of the lower depth.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having an optimumshape and an optimum size for each maximum coding unit, based on thesize of the maximum coding unit and the maximum depth determinedconsidering characteristics of the current picture. Also, since encodingmay be performed on each maximum coding unit by using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having high resolution or large data amount is encodedin a related art macroblock, a number of macroblocks per pictureexcessively increases. Accordingly, a number of pieces of compressedinformation generated for each macroblock increases, and thus it isdifficult to transmit the compressed information and data compressionefficiency decreases. However, by using the video encoding apparatus100, image compression efficiency may be increased since a coding unitis adjusted while considering characteristics of an image whileincreasing a maximum size of a coding unit while considering a size ofthe image.

FIG. 2 is a block diagram of a video decoding apparatus 200, accordingto an exemplary embodiment.

The video decoding apparatus 200 includes a receiver 210, an image dataand encoding information extractor 220, and an image data decoder 230.Definitions of various terms, such as a coding unit, a depth, aprediction unit, a transformation unit, and information about variousencoding modes, for various operations of the video decoding apparatus200 are identical to those described with reference to FIG. 1 and thevideo encoding apparatus 100.

The receiver 210 receives and parses a bitstream of an encoded video.The image data and encoding information extractor 220 extracts encodedimage data for each coding unit from the parsed bitstream, wherein thecoding units have a tree structure according to each maximum codingunit, and outputs the extracted image data to the image data decoder230. The image data and encoding information extractor 220 may extractinformation about a maximum size of a coding unit of a current picture,from a header about the current picture.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth and an encoding mode for the codingunits having a tree structure according to each maximum coding unit,from the parsed bitstream. The extracted information about the codeddepth and the encoding mode is output to the image data decoder 230. Inother words, the image data in a bit stream is split into the maximumcoding unit so that the image data decoder 230 decodes the image datafor each maximum coding unit.

The information about the coded depth and the encoding mode according tothe maximum coding unit may be set for information about at least onecoding unit corresponding to the coded depth, and information about anencoding mode may include information about a partition type of acorresponding coding unit corresponding to the coded depth, about aprediction mode, and a size of a transformation unit. Also, splittinginformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according toeach maximum coding unit extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to generate a minimum encoding error when anencoder, such as the video encoding apparatus 100, repeatedly performsencoding for each deeper coding unit according to depths according toeach maximum coding unit. Accordingly, the video decoding apparatus 200may restore an image by decoding the image data according to a codeddepth and an encoding mode that generates the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract the information about thecoded depth and the encoding mode according to the predetermined dataunits. If information about a coded depth and encoding mode of acorresponding maximum coding unit is recorded according to predetermineddata units, the predetermined data units to which the same informationabout the coded depth and the encoding mode is assigned may be inferredto be the data units included in the same maximum coding unit.

The image data decoder 230 restores the current picture by decoding theimage data in each maximum coding unit based on the information aboutthe coded depth and the encoding mode according to the maximum codingunits. In other words, the image data decoder 230 may decode the encodedimage data based on the extracted information about the partition type,the prediction mode, and the transformation unit for each coding unitfrom among the coding units having the tree structure included in eachmaximum coding unit. A decoding process may include a predictionincluding intra prediction and motion compensation, and an inversetransformation.

The image data decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

Also, the image data decoder 230 may perform inverse transformationaccording to each transformation unit in the coding unit, based on theinformation about the size of the transformation unit of the coding unitaccording to coded depths, so as to perform the inverse transformationaccording to maximum coding units.

The image data decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. If the split information indicates that image data is no longersplit in the current depth, the current depth is a coded depth.Accordingly, the image data decoder 230 may decode encoded data of atleast one coding unit corresponding to the each coded depth in thecurrent maximum coding unit by using the information about the partitiontype of the prediction unit, the prediction mode, and the size of thetransformation unit for each coding unit corresponding to the codeddepth.

In other words, data units containing the encoding information includingthe same split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by theimage data decoder 230 in the same encoding mode.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each maximum coding unit, and may use theinformation to decode the current picture. In other words, the codingunits having the tree structure determined to be the optimum codingunits in each maximum coding unit may be decoded.

Accordingly, even if image data has high resolution and a large amountof data, the image data may be efficiently decoded and restored by usinga size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, by usinginformation about an optimum encoding mode received from an encoder.

A method of determining coding units having a tree structure, aprediction unit, and a transformation unit, according to an exemplaryembodiment, will now be described with reference to FIGS. 3 through 13.

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment.

A size of a coding unit may be expressed in width×height, and may be64×64, 32×32, 16×16, and 8×8. A coding unit of 64×64 may be split intopartitions of 64×64, 64×32, 32×64, or 32×32, and a coding unit of 32×32may be split into partitions of 32×32, 32×16, 16×32, or 16×16, a codingunit of 16×16 may be split into partitions of 16×16, 16×8, 8×16, or 8×8,and a coding unit of 8×8 may be split into partitions of 8×8, 8×4, 4×8,or 4×4.

In video data 310, a resolution is 1920×1080, a maximum size of a codingunit is 64, and a maximum depth is 2. In video data 320, a resolution is1920×1080, a maximum size of a coding unit is 64, and a maximum depth is3. In video data 330, a resolution is 352×288, a maximum size of acoding unit is 16, and a maximum depth is 1. The maximum depth shown inFIG. 3 denotes a total number of splits from a maximum coding unit to aminimum decoding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havingthe higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe vide data 310 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened to two layers by splitting the maximum coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a maximum coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened to one layer by splitting the maximumcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened to 3 layers by splitting the maximumcoding unit three times. As a depth deepens, detailed information may beprecisely expressed.

FIG. 4 is a block diagram of an image encoder 400 based on coding units,according to an exemplary embodiment.

The image encoder 400 performs operations of the coding unit determiner120 of the video encoding apparatus 100 to encode image data. In otherwords, an intra predictor 410 performs intra prediction on coding unitsin an intra mode, from among a current frame 405, and a motion estimator420 and a motion compensator 425 performs inter estimation and motioncompensation on coding units in an inter mode from among the currentframe 405 by using the current frame 405, and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 and a loopfiltering unit 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

In order for the image encoder 400 to be applied in the video encodingapparatus 100, all elements of the image encoder 400, i.e., the intrapredictor 410, the motion estimator 420, the motion compensator 425, thetransformer 430, the quantizer 440, the entropy encoder 450, the inversequantizer 460, the inverse transformer 470, the deblocking unit 480, andthe loop filtering unit 490 perform operations based on each coding unitfrom among coding units having a tree structure while considering themaximum depth of each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determines partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransformation unit in each coding unit from among the coding unitshaving a tree structure.

FIG. 5 is a block diagram of an image decoder 500 based on coding units,according to an exemplary embodiment.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding from a bitstream 505. The encodedimage data is output as inverse quantized data through an entropydecoder 520 and an inverse quantizer 530, and the inverse quantized datais restored to image data in a spatial domain through an inversetransformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 and a loop filtering unit 580. Also, the image data that ispost-processed through the deblocking unit 570 and the loop filteringunit 580 may be output as the reference frame 585.

In order to decode the image data in the image data decoder 230 of thevideo decoding apparatus 200, the image decoder 500 may performoperations that are performed after the parser 510.

In order for the image decoder 500 to be applied in the video decodingapparatus 200, all elements of the image decoder 500, i.e., the parser510, the entropy decoder 520, the inverse quantizer 530, the inversetransformer 540, the intra predictor 550, the motion compensator 560,the deblocking unit 570, and the loop filtering unit 580 performoperations based on coding units having a tree structure for eachmaximum coding unit.

Specifically, the intra prediction 550 and the motion compensator 560perform operations based on partitions and a prediction mode for each ofthe coding units having a tree structure, and the inverse transformer540 perform operations based on a size of a transformation unit for eachcoding unit.

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

The video encoding apparatus 100 and the video decoding apparatus 200use hierarchical coding units so as to consider characteristics of animage. A maximum height, a maximum width, and a maximum depth of codingunits may be adaptively determined according to the characteristics ofthe image, or may be differently set by a user. Sizes of deeper codingunits according to depths may be determined according to thepredetermined maximum size of the coding unit.

In a hierarchical structure 600 of coding units, according to anexemplary embodiment, the maximum height and the maximum width of thecoding units are each 64, and the maximum depth is 4. Since a depthdeepens along a vertical axis of the hierarchical structure 600, aheight and a width of the deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a minimum coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, ifthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsinclude in the encoding unit 610, i.e. a partition 610 having a size of64×64, partitions 612 having the size of 64×32, partitions 614 havingthe size of 32×64, or partitions 616 having the size of 32×32.

Similarly, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 1 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having a size of 32×32, partitions622 having a size of 32×16, partitions 624 having a size of 16×32, andpartitions 626 having a size of 16×16.

Similarly, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 2 may be split into partitions included in thecoding unit 630, i.e. a partition having a size of 16×16 included in thecoding unit 630, partitions 632 having a size of 16×8, partitions 634having a size of 8×16, and partitions 636 having a size of 8×8.

Similarly, a prediction unit of the coding unit 640 having the size of8×8 and the depth of 3 may be split into partitions included in thecoding unit 640, i.e. a partition having a size of 8×8 included in thecoding unit 640, partitions 642 having a size of 8×4, partitions 644having a size of 4×8, and partitions 646 having a size of 4×4.

The coding unit 650 having the size of 4×4 and the depth of 4 is theminimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is only assigned to a partitionhaving a size of 4×4.

In order to determine the at least one coded depth of the coding unitsconstituting the maximum coding unit 610, the coding unit determiner 120of the video encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the maximum coding unit 610.

A number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding for a current depth from among the depths,a least encoding error may be selected for the current depth byperforming encoding for each prediction unit in the coding unitscorresponding to the current depth, along the horizontal axis of thehierarchical structure 600. Alternatively, the minimum encoding errormay be searched for by comparing the least encoding errors according todepths, by performing encoding for each depth as the depth deepens alongthe vertical axis of the hierarchical structure 600. A depth and apartition having the minimum encoding error in the coding unit 610 maybe selected as the coded depth and a partition type of the coding unit610.

FIG. 7 is a diagram for describing a relationship between a coding unit710 and transformation units 720, according to an exemplary embodiment.

The video encoding apparatus 100 or the video decoding apparatus 200encodes or decodes an image according to coding units having sizessmaller than or equal to a maximum coding unit for each maximum codingunit. Sizes of transformation units for transformation during encodingmay be selected based on data units that are not larger than acorresponding coding unit.

For example, in the video encoding apparatus 100 or the video decodingapparatus 200, if a size of the coding unit 710 is 64×64, transformationmay be performed by using the transformation units 720 having a size of32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transformationunits having the size of 32×32, 16×16, 8×8, and 4×4, which are smallerthan 64×64, and then a transformation unit having the least coding errormay be selected.

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

The output unit 130 of the video encoding apparatus 100 may encode andtransmit information 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transformationunit for each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, a current coding unit CU_0 having a size of2N×2N may be split into any one of a partition 802 having a size of2N×2N, a partition 804 having a size of 2N×N, a partition 806 having asize of N×2N, and a partition 808 having a size of N×N. Here, theinformation 800 about a partition type is set to indicate one of thepartition 804 having a size of 2N×N, the partition 806 having a size ofN×2N, and the partition 808 having a size of N×N

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transformation unit to be based on whentransformation is performed on a current coding unit. For example, thetransformation unit may be a first intra transformation unit 822, asecond intra transformation unit 824, a first inter transformation unit826, or a second intra transformation unit 828.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information 800, 810, and820 for decoding, according to each deeper coding unit.

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding a coding unit 900 having adepth of 0 and a size of 2N_0×2N_0 may include partitions of a partitiontype 912 having a size of 2N_0×2N_0, a partition type 914 having a sizeof 2N_0×N_0, a partition type 916 having a size of N_0×2N_0, and apartition type 918 having a size of N_0×N_0. FIG. 9 only illustrates thepartition types 912 through 918 which are obtained by symmetricallysplitting the prediction unit 910, but a partition type is not limitedthereto, and the partitions of the prediction unit 910 may includeasymmetrical partitions, partitions having a predetermined shape, andpartitions having a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_0×2N_0, two partitions having a size of 2N_0×N_0, twopartitions having a size of N_0×2N_0, and four partitions having a sizeof N_0×N_0, according to each partition type. The prediction encoding inan intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_0×2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

If an encoding error is smallest in one of the partition types 912through 916, the prediction unit 910 may not be split into a lowerdepth.

If the encoding error is the smallest in the partition type 918, a depthis changed from 0 to 1 to split the partition type 918 in operation 920,and encoding is repeatedly performed on coding units 930 having a depthof 2 and a size of N_0×N_0 to search for a minimum encoding error.

A prediction unit 940 for prediction encoding the coding unit 930 havinga depth of 1 and a size of 2N_1×2N_1 (=N_0×N_0) may include partitionsof a partition type 942 having a size of 2N_1×2N_1, a partition type 944having a size of 2N_1×N_1, a partition type 946 having a size ofN_1×2N_1, and a partition type 948 having a size of N_1×N_1.

If an encoding error is the smallest in the partition type 948, a depthis changed from 1 to 2 to split the partition type 948 in operation 950,and encoding is repeatedly performed on coding units 960, which have adepth of 2 and a size of N_2×N_2 to search for a minimum encoding error.

When a maximum depth is d, split operation according to each depth maybe performed up to when a depth becomes d−1, and split information maybe encoded as up to when a depth is one of 0 to d−2. In other words,when encoding is performed up to when the depth is d−1 after a codingunit corresponding to a depth of d−2 is split in operation 970, aprediction unit 990 for prediction encoding a coding unit 980 having adepth of d−1 and a size of 2N_(d−1)×2N_(d−1) may include partitions of apartition type 992 having a size of 2N_(d−1)×2N_(d−1), a partition type994 having a size of 2N_(d−1)×N_(d−1), a partition type 996 having asize of N_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d−1)×2N_(d−1), two partitions having a size of2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), fourpartitions having a size of N_(d−1)×N_(d−1) from among the partitiontypes 992 through 998 to search for a partition type having a minimumencoding error.

Even when the partition type 998 has the minimum encoding error, since amaximum depth is d, a coding unit CU_(d−1) having a depth of d−1 is nolonger split to a lower depth, and a coded depth for the coding unitsconstituting a current maximum coding unit 900 is determined to be d−1and a partition type of the current maximum coding unit 900 may bedetermined to be N_(d−1)×N_(d−1). Also, since the maximum depth is d anda minimum coding unit 980 having a lowermost depth of d−1 is no longersplit to a lower depth, split information for the minimum coding unit980 is not set.

A data unit 999 may be a ‘minimum unit’ for the current maximum codingunit. A minimum unit according to an exemplary embodiment may be asquare data unit obtained by splitting a minimum coding unit 980 by 4.By performing the encoding repeatedly, the video encoding apparatus 100may select a depth having the least encoding error by comparing encodingerrors according to depths of the coding unit 900 to determine a codeddepth, and set a corresponding partition type and a prediction mode asan encoding mode of the coded depth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, only split informationof the coded depth is set to 0, and split information of depthsexcluding the coded depth is set to 1.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information about thecoded depth and the prediction unit of the coding unit 900 to decode thepartition 912. The video decoding apparatus 200 may determine a depth,in which split information is 0, as a coded depth by using splitinformation according to depths, and use information about an encodingmode of the corresponding depth for decoding.

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transformation units 1070,according to an exemplary embodiment.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the video encoding apparatus100, in a maximum coding unit. The prediction units 1060 are partitionsof prediction units of each of the coding units 1010, and thetransformation units 1070 are transformation units of each of the codingunits 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some encoding units 1014, 1016, 1022,1032, 1048, 1050, 1052, and 1054 are obtained by splitting the codingunits in the encoding units 1010. In other words, partition types in thecoding units 1014, 1022, 1050, and 1054 have a size of 2N×N, partitiontypes in the coding units 1016, 1048, and 1052 have a size of N×2N, anda partition type of the coding unit 1032 has a size of N×N. Predictionunits and partitions of the coding units 1010 are smaller than or equalto each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transformation units 1070 in a data unitthat is smaller than the coding unit 1052. Also, the coding units 1014,1016, 1022, 1032, 1048, 1050, and 1052 in the transformation units 1070are different from those in the prediction units 1060 in terms of sizesand shapes. In other words, the video encoding and decoding apparatuses100 and 200 may perform intra prediction, motion estimation, motioncompensation, transformation, and inverse transformation individually ona data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit. Table 1 shows the encodinginformation that may be set by the video encoding and decodingapparatuses 100 and 200.

TABLE 1 Split Information 0 (Encoding on Coding Unit having Size of 2N ×2N and Current Depth of d) Prediction Split Mode Partition Type Size ofTransformation Unit Information 1 Intra Symmetrical Asymmetrical SplitSplit Repeatedly Inter Partition Partition Information 0 Information 1Encode Type Type of of Coding Units Transformation Transformation havingLower Unit Unit Depth of d + 1 Skip 2N × 2N 2N × nU 2N × 2N N × N (Only2N × N  2N × nD (Symmetrical 2N x 2N)  N × 2N nL × 2N Type) N × N nR ×2N N/2 × N/2 (Asymmetrical Type)

The output unit 130 of the video encoding apparatus 100 may output theencoding information about the coding units having a tree structure, andthe image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract the encoding information about thecoding units having a tree structure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth, in which a current coding unit is no longer split into alower depth, is a coded depth, and thus information about a partitiontype, prediction mode, and a size of a transformation unit may bedefined for the coded depth. If the current coding unit is further splitaccording to the split information, encoding is independently performedon four split coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or width of the prediction unit. The asymmetrical partition typeshaving the sizes of 2N×nU and 2N×nD may be respectively obtained bysplitting the height of the prediction unit in 1:3 and 3:1, and theasymmetrical partition types having the sizes of nL×2N and nR×2N may berespectively obtained by splitting the width of the prediction unit in1:3 and 3:1

The size of the transformation unit may be set to be two types in theintra mode and two types in the inter mode. In other words, if splitinformation of the transformation unit is 0, the size of thetransformation unit may be 2N×2N, which is the size of the currentcoding unit. If split information of the transformation unit is 1, thetransformation units may be obtained by splitting the current codingunit. Also, if a partition type of the current coding unit having thesize of 2N×2N is a symmetrical partition type, a size of atransformation unit may be N×N, and if the partition type of the currentcoding unit is an asymmetrical partition type, the size of thetransformation unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, if a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoded information of the data units,and the searched adjacent coding units may be referred for predictingthe current coding unit.

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transformation unit,according to encoding mode information of Table 1.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

When the partition type is set to be symmetrical, i.e. the partitiontype 1322, 1324, 1326, or 1328, a transformation unit 1342 having a sizeof 2N×2N is set if split information (TU size flag) of a transformationunit is 0, and a transformation unit 1344 having a size of N×N is set ifa TU size flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transformation unit 1352 having a sizeof 2N×2N is set if a TU size flag is 0, and a transformation unit 1354having a size of N/2×N/2 is set if a TU size flag is 1.

Hereinafter, intra prediction performed on a prediction unit by theintra predictor 410 of the video encoding apparatus 100 of FIG. 4 andthe intra predictor 550 of the video decoding apparatus 200 of FIG. 5will be described in detail.

The intra predictors 410 and 550 perform intra prediction for obtaininga prediction value of a current prediction unit by using adjacent pixelsof the current prediction unit. Considering that a prediction unit has asize equal to or higher than 16×16, the intra predictors 410 and 550additionally performs an intra prediction mode having variousdirectivities using a (dx, dy) parameter as well as an intra predictionmode having a limited directivity according to a related art. The intraprediction mode having various directivities according to an exemplaryembodiment will be described later in detail.

Also, in order to obtain a predictor of a current pixel, the intrapredictors 410 and 550 may generate a predictor P1 via linearinterpolation in a horizontal direction of a current pixel and apredictor P2 via linear interpolation in a vertical direction of thecurrent pixel, and use an average value of the predictors P1 and P2 as apredictor of the current pixel. An intra prediction mode for generatinga predictor of a current pixel by combining predictors obtained vialinear interpolation in a horizontal direction and linear interpolationin a vertical direction is defined as a planar mode. Specifically, theintra predictors 410 and 550 generate a virtual pixel used in linearinterpolation in a horizontal direction by using at least one adjacentpixel located upper right to a current prediction unit and a virtualpixel used in linear interpolation in a vertical direction by using atleast one adjacent pixel located lower left to the current predictionunit in a planar mode. The planar mode according to an exemplaryembodiment will be described in detail later.

FIG. 14 is a table showing a number of intra prediction modes accordingto a size of a prediction unit, according to an exemplary embodiment.

The intra predictors 410 and 550 may variously set the number of intraprediction modes to be applied to the prediction unit according to thesize of the prediction unit. For example, referring to FIG. 14, when thesize of the prediction unit to be intra predicted is N×N, the numbers ofintra prediction modes actually performed on the prediction units havingthe sizes of 2×2, 4×4, 8×8, 16×16, 32×32, 64×64, and 128×128 may berespectively set to 5, 9, 9, 17, 33, 5, and 5 in Example 2. The numberof intra prediction modes actually performed differs according to thesize of the prediction unit because overhead for encoding predictionmode information differs according to the size of the prediction unit.In other words, even though a portion of a prediction unit occupying anentire image is small, overhead for transmitting additional information,such as a prediction mode of such a small prediction unit may be large.Accordingly, when a prediction unit having a small size is encoded inmany prediction modes, an amount of bits may increase and thuscompression efficiency may decrease. Also, since a prediction unithaving a large size, for example, a prediction unit having a size equalto or larger than 64×64, is generally mostly selected as a predictionunit of a flat region of an image, it may be insufficient in terms ofcompression efficiency to encode the prediction unit having a largesize, which is mostly selected to encode a flat region, in manyprediction modes. Accordingly, when a size of prediction unit is toolarge or too small compared to a predetermined size, a relatively smallnumber of intra prediction modes may be applied. However, the number ofintra prediction modes applied according to the size of a predictionunit is not limited to FIG. 14, and may vary. The number of intraprediction modes applied according to the size of a prediction unit, asshown in FIG. 14, is only an example, and may vary. Alternatively, thenumber of intra prediction modes applied to the prediction unit may bealways uniform regardless of the size of a prediction unit.

The intra predictors 410 and 550 may include, as an intra predictionmode applied to a prediction unit, an intra prediction mode thatdetermines an adjacent reference pixel by using a line having apredetermined angle based on a pixel in a prediction unit and using thedetermined adjacent reference pixel as a predictor of the pixel. Theangle of such a line may be set by using a parameter (dx, dy), whereindx and dy are each an integer. For example, when 33 prediction modes arerespectively defined to be modes N, wherein N is an integer from 0 to32, a mode 0 is set to a vertical mode, a mode 1 is set to a horizontalmode, a mode 2 is set to a DC mode, a mode 3 is set to a plane mode, anda mode 32 is set to a planar mode. Also, modes 4 through 31 may bedefined to be intra prediction modes determining an adjacent referencepixel by using a line having a directivity of tan−1(dy/dx) using (dx,dy) respectively expressed by (1,−1), (1,1), (1,2), (2,1), (1,−2),(2,1), (1,−2), (2,−1), (2,−11), (5,−7), (10,−7), (11,3), (4,3), (1,11),(1,−1), (12,−3), (1,−11), (1,−7), (3,−10), (5,−6), (7,−6), (7,−4),(11,1), (6,1), (8,3), (5,3), (5,7), (2,7), (5,−7), and (4,−3) of Table1, and using the determined adjacent reference pixel for intraprediction.

TABLE 2 mode # dx dy mode 4 1 −1 mode 5 1 1 mode 6 1 2 mode 7 2 1 mode 81 −2 mode 9 2 −1 mode 10 2 −11 mode 11 5 −7 mode 12 10 −7 mode 13 11 3mode 14 4 3 mode 15 1 11 mode 16 1 −1 mode 17 12 −3 mode 18 1 −11 mode19 1 −7 mode 20 3 −10 mode 21 5 −6 mode 22 7 −6 mode 23 7 −4 mode 24 111 mode 25 6 1 mode 26 8 3 mode 27 5 3 mode 28 5 7 mode 29 2 7 mode 30 5−7 mode 31 4 −3 mode 0 is vertical mode, mode 1 is horizontal mode, mode2 is DC mode, mode 3 is plane mode, and mode 32 is planar mode.

The number of intra prediction modes used by the intra predictors 410and 550 is not limited to Table 2, and may vary based on whether acurrent prediction unit is a chrominance component or luminancecomponent or based on a size of current prediction unit. Also, each modeN may denote an intra prediction mode different from above. For example,the number of intra prediction modes may be 36, wherein a mode 0 is aplanar mode described later, a mode 1 is a DC mode, modes 2 through 34are intra prediction modes having 33 directivities as described later,and a mode 35 is an intra prediction mode Intra_FromLuma using aprediction unit in a luminance component corresponding to a predictionunit in a chrominance component. The mode 35, i.e., the intra predictionmode Intra_FromLuma using the prediction unit in the luminance componentcorresponding to the prediction unit in the chrominance component isonly applied to the prediction unit in the chrominance component, and isnot used to intra predict the prediction unit in the luminancecomponent.

FIG. 15 is a reference diagram for describing intra prediction modeshaving various directivities, according to an exemplary embodiment.

As described above, the intra predictors 410 and 550 may determine anadjacent reference pixel by using a line having an angle of tan⁻¹(dy/dx)determined by a plurality of (dx, dy) parameters, and perform intraprediction by using the determined adjacent reference pixel.

Referring to FIG. 15, adjacent pixels A and B located on an extensionline 150 having an angle of tan⁻¹(dy/dx) determined according to a valueof (dx, dy) according to the intra prediction modes of Table 2 based ona current pixel P to be predicted in a current prediction unit may beused as predictors of the current pixel P. Here, an adjacent pixel usedas a predictor may be a pixel of a previous prediction unit that ispre-encoded and pre-restored and is located either above, left, upperright, or lower left of a current prediction unit. As such, byperforming prediction encoding according to intra prediction modeshaving various directivities, compression may be effectively performedaccording to characteristics of an image.

In FIG. 15, when a predictor of the current pixel P is generated byusing an adjacent pixel located on or near the extension line 150, theextension line 150 actually has a directivity of tan⁻¹(dy/dx) and adivision of (dy/dx) is required to determine the adjacent pixel usingthe extension line 150, and thus hardware or software may include adecimal point operation, thereby increasing a throughput. Accordingly,when a prediction direction for selecting a reference pixel is set byusing (dx, dy) parameters, dx and dy may be set to decrease athroughput.

FIG. 16 is a diagram for describing a relationship between a currentpixel and adjacent pixels disposed on an extension line having adirectivity of (dx, dy), according to an exemplary embodiment.

Referring to FIG. 16, P 1610 denotes the current pixel located at (j, i)and A 1611 and B 1612 respectively denote an adjacent upper pixel and anadjacent left pixel located on an extension line having a directivity,i.e., an angle of tan⁻¹(dy/dx), passing through the current pixel P1610. It is assumed that a size of a prediction unit including thecurrent pixel P 1610 is nS×nS wherein nS is a positive integer, alocation of pixel of the prediction unit is one of (0, 0) to (nS−1,nS−1), a location of the adjacent upper pixel A 1611 on an x-axis is (m,−1) wherein m is an integer, and a location of the adjacent left pixel B1612 on an y-axis is (−1, n) wherein n is an integer. The location ofthe adjacent upper pixel A 1611 meeting the extension line passingthrough the current pixel P1610 is (j+i*dx/dy, −1), and the location ofthe adjacent left pixel B 1612 is (−1, i+j*dy/dx). Accordingly, in orderto determine the adjacent upper pixel A 1611 or adjacent left pixel B1612 to predict the current pixel P1610, a division operation, such asdx/dy or dy/dx is required. As described above, since operationcomplexity of the division operation is high, an operation speed insoftware or hardware may be low. Accordingly, at least one of dx and dyindicating a directivity of a prediction mode for determining anadjacent pixel may be a power of 2. In other words, when n and m areeach an integer, dx and dy may be respectively 2^n and 2^m.

When the adjacent left pixel B 1612 is used as a predictor of thecurrent pixel P 1610 and dx has a value of 2^n, an j*dy/dx operationrequired to determine (−1, i+j*dy/dx), i.e., a location of the adjacentleft pixel B 1612, may be (i*dy)/(2^n) and a division operation using apower of 2 may be realized via a shift operation, such as (i*dy)>>n, andthus a throughput is decreased.

Similarly, when the adjacent upper pixel A 1611 is used as a predictorof the current pixel P 1610 and dy has a value of 2^m, an i*dx/dyoperation required to determine (j+i*dx/dy,−1), i.e., a location of theadjacent upper pixel A 1611 may be (i*dx)/(2^m) and a division operationusing a power of 2 may be realized via a shift operation, such as(i*dx)>>m.

FIGS. 17 and 18 are diagrams showing directions of an intra predictionmode, according to exemplary embodiments.

Generally, straight line patterns shown in an image or video signal aremostly vertical or horizontal. Thus, when an intra prediction modehaving various directivities is defined by using a (dx, dy) parameter,values of dx and dy may be defined as follows to improve encodingefficiency of an image.

In detail, when dy has a fixed value of 2^m, an absolute value of dx maybe set such that an interval between prediction directions close to avertical direction is narrow and an interval between prediction modesincreases towards a prediction direction close to a horizontaldirection. For example, referring to FIG. 17, when dy is 2^5, i.e., 32,dx may be set to 2, 5, 9, 13, 17, 21, 26, 32, −2, −5, −9, −13, −17, −21,−26, and −32 such that an interval between prediction directions closeto a vertical direction is relatively narrow and an interval betweenprediction modes increases towards a prediction direction close to ahorizontal direction.

Similarly, when dx has a fixed value of 2^n, an absolute value of dy maybe set such that an interval between prediction directions close to ahorizontal direction is narrow and an interval between prediction modesincreases towards a prediction direction close to a horizontaldirection. For example, referring to FIG. 18, when dx is 2^5, i.e., 32,dy may be set to 2, 5, 9, 13, 17, 21, 26, 32, −2, −5, −9, −13, −17, −21,−26, and −32 such that an interval between prediction directions closeto a horizontal direction is relatively narrow and an interval betweenprediction modes increase towards a prediction direction close to avertical direction.

Also, when one of values of dx and dy is fixed, the other value may beset such as to increase according to prediction modes. For example, whenthe value of dy is fixed, an interval between values of dx may be set toincrease by a predetermined value. Such an increment may be setaccording to angles divided between a horizontal direction and avertical direction. For example, when dy is fixed, dx may have anincrement a in a section where an angle with a vertical axis is smallerthan 15°, an increment b in a section where the angle is between 15° and30°, and an increment c in a section where the angle is higher than 30°.

For example, prediction modes having directivities of tan⁻¹(dy/dx) using(dx, dy) may be defined by (dx, dy) parameters shown in Tables 3 through5.

TABLE 3 dx dy −32 32 −26 32 −21 32 −17 32 −13 32 −9 32 −5 32 −2 32 0 322 32 5 32 9 32 13 32 17 32 21 32 26 32 32 32 32 −26 32 −21 32 −17 32 −1332 −9 32 −5 32 −2 32 0 32 2 32 5 32 9 32 13 32 17 32 21 32 26 32 32

TABLE 4 dx dy -32 32 -25 32 -19 32 -14 32 -10 32 -6 32 -3 32 -1 32 0 321 32 3 32 6 32 10 32 14 32 19 32 25 32 32 32 32 -25 32 -19 32 -14 32 -1032 -6 32 -3 32 -1 32 0 32 1 32 3 32 6 32 10 32 14 32 19 32 25 32 32

TABLE 5 dx dy -32 32 -27 32 -23 32 -19 32 -15 32 -11 32 -7 32 -3 32 0 323 32 7 32 11 32 15 32 19 32 23 32 27 32 32 32 32 -27 32 -23 32 -19 32-15 32 -11 32 -7 32 -3 32 0 32 3 32 7 32 11 32 15 32 19 32 23 32 27 3232

As described above, the intra prediction modes using (dx, dy) parametersuse the adjacent left pixel (−1, i+j*dy/dx) or the adjacent upper pixel(j+i*dx/dy,−1) as a predictor of a pixel located at (j,i). When at leastone of dx and dy has a power of 2 as shown in Table 2, locations of theadjacent left pixel (−1, i+j*dy/dx) and adjacent upper pixel(j+i*dx/dy,−1) may be obtained via only multiplication and shiftoperations without a division operation. When dx is 2^n, i.e., 32, in(dx, dy) as shown in Table 2, a division operation using dx may bereplaced by a right shift operation, and thus a location of an adjacentleft pixel may be obtained without a division operation based on(i*dy)>>n. Similarly, when dy is 2^m, i.e., 32, in (dx, dy) as shown inTable 2, a division operation using dx may be replaced by a right shiftoperation, and thus a location of an adjacent upper pixel may beobtained without a division operation based on (i*dx)>>m.

FIG. 19 is a diagram showing directions of an intra prediction modehaving 33 directivities, according to an exemplary embodiment.

The intra predictors 410 and 550 may determine an adjacent pixel to beused as a predictor of a current pixel according to intra predictionmodes having 33 directivities shown in FIG. 19. As described above,directions of intra prediction modes may be set such that an intervalbetween prediction modes decreases towards a horizontal or verticaldirection and increases farther from a vertical or horizontal direction.

FIGS. 20A and 20B are diagrams for describing a planar mode according toexemplary embodiments.

As described above, the intra predictors 410 and 550 generate, in aplanar mode, a virtual pixel used in linear interpolation in ahorizontal direction by using at least one adjacent pixel located upperright to a current prediction unit and generates a virtual pixel used inlinear interpolation in a vertical direction by using at least oneadjacent pixel located lower left to the current prediction unit. Also,the intra predictors 410 and 550 generate a prediction value of acurrent pixel by using an average value of two predictors generated vialinear interpolation in horizontal and vertical directions using thevirtual pixels and adjacent pixels.

Referring to FIG. 20A, the intra predictors 410 and 550 obtain a firstvirtual pixel 2012 located on the same line as a current predicted pixel2011 in a current prediction unit 2010 and corresponding to a pixellocated farthest right of the current prediction unit 2010 by using atleast one adjacent pixel 2020 located upper right to the currentprediction unit 2010. The number of adjacent pixels 2020 used to obtainthe first virtual pixel 2012 may be pre-determined. For example, theintra predictors 410 and 550 may determine a value generated by using anaverage value or weighted average value of a T1 2021 and a T2 2022,which are initial two upper right adjacent pixels, as the first virtualpixel 2012.

Also, the intra predictors 410 and 550 may determine the number ofadjacent pixels 2020 used to obtain the first virtual pixel 2012 basedon the size of the current prediction unit 2010. For example, when thesize of the current prediction unit 2010 is nS×nS wherein nS is aninteger, the intra predictors 410 and 550 may select nS/(2^m) upperright adjacent pixels from among the adjacent pixels 2020 used to obtainthe first virtual pixel 2012, wherein m is in integer satisfying acondition that 2^m is not higher than nS, and obtain the first virtualpixel 2012 by using an average value or weighted average value of theselected upper right adjacent pixels. In other words, the intrapredictors 410 and 550 may select nS/2, nS/4, nS/8, and so on, pixelsfrom among the adjacent pixels 2020. For example, when the size of thecurrent prediction unit 2010 is 32×32, the intra predictors 410 and 550may select 32/2, 32/4, 32/8, 32/16, 32/32, i.e., 1 to 16 upper rightadjacent pixels.

Similarly, referring to FIG. 20B, the intra predictors 410 and 550obtains a second virtual pixel 2014 located on the same column as thecurrent predicted pixel 2011 in the current prediction unit 2010 andcorresponding to a pixel located farthest below the current predictionunit 2010 by using at least one adjacent pixel 2030 located lower leftto the current prediction unit 2010. The number of adjacent pixels 2030used to obtain the second virtual pixel 2014 may be pre-determined. Forexample, a value generated by using an average value or weighted averagevalue of L1 2031 and L2 2032, which are two initial lower left adjacentpixels, may be determined as the second virtual pixel 2014.

Also, the intra predictors 410 and 550 may determine the number ofadjacent pixels 2030 used to obtain the second virtual pixel 2014 basedon the size of the current prediction unit 2010. As described above,when the size of the current prediction unit 2010 is nS×nS wherein nS isan integer, the intra predictors 410 and 550 may select nS/(2^m) lowerleft adjacent pixels from among the adjacent pixels 2030 used to obtainthe second virtual pixel 2014, wherein m is an integer satisfying acondition that 2^m is not higher than nS, and obtain the second virtualpixel 2014 by using an average value or weighted average value of theselected lower left adjacent pixels.

Meanwhile, if the adjacent pixels 2020 are not usable by being includedin a prediction unit encoded after the current prediction unit 2010, theintra predictors 410 and 550 may use a pixel T0 immediately left of theadjacent pixels 2020 as the first virtual pixel 2012. On the other hand,if the adjacent pixels 2030 are not usable by being included in aprediction unit encoded after the current prediction unit 2010, theintra predictors 410 and 550 may use a pixel L0 immediately above theadjacent pixels 2030 as the second virtual pixel 2014.

Referring back to FIG. 20A, the intra predictors 410 and 550 generate afirst prediction value p1 of the current predicted pixel 2011 byperforming linear interpolation using a geometric average valueconsidering a distance between the current predicted pixel 2011 and thefirst virtual pixel 2012 obtained from the adjacent pixels 2020 and adistance between the current predicted pixel 2011 and an adjacent leftpixel 2013 on the same line as the current predicted pixel 2011.

When a pixel value of the adjacent left pixel 2013 is rec(−1,y), a pixelvalue of the first virtual pixel 2012 located at (nS−1,y) is T wherein Tis a real number, and a prediction value of the current predicted pixel2011 is p(x,y) wherein x,y=0 to nS−1, wherein (x,y) denotes a locationof the current predicted pixel 2011 of the current prediction unit 2010and rec(x,y) denotes adjacent pixels of the current prediction unit 2010wherein (x,y=−1 to 2*nS−1), a first prediction value p1 (x,y) may beobtained according to an equation p1 (x,y)=(nS−1−x)*rec(−1,y)+(x+1)*T.Here, (ns−1−x) corresponds to a distance between the current predictedpixel 2011 and the first virtual pixel 2012 and (x+1) corresponds to adistance between the current predicted pixel 2011 and the adjacent leftpixel 2013. As such, the intra predictors 410 and 550 generate the firstprediction value p1 through linear interpolation using the distancebetween the first virtual pixel 2012 and the current predicted pixel2011, the distance between the current predicted pixel 2011 and theadjacent left pixel 2013 on the same line as the current predicted pixel2011, the pixel value of the first virtual pixel 2012, and the pixelvalue of the adjacent left pixel 2013.

Referring back to FIG. 20B, the intra predictors 410 and 550 generate asecond prediction value p2 of the current predicted pixel 2011 byperforming linear interpolation using a geometric average valueconsidering a distance between the current predicted pixel 2011 and thesecond virtual pixel 2014 obtained from the adjacent pixels 2030 and adistance between the current predicted pixel 2011 and an adjacent upperpixel 2015 on the same column as the current predicted pixel 2011.

When a pixel value of the adjacent upper pixel 2015 is rec(x,−1), apixel value of the second virtual pixel 2014 located at (x,nS−1) is Lwherein L is a real number, and a prediction value of the currentpredicted pixel 2011 is p(x,y) wherein x,y=0 to nS−1, wherein (x,y)denotes a location of the current predicted pixel 2011 of the currentprediction unit 2010 and rec(x,y) denotes adjacent pixels of the currentprediction unit 2010 wherein (x,y=−1 to 2*nS−1), a second predictionvalue p2 (x,y) may be obtained according to an equation p2(x,y)=(nS−1−y)*rec(x,−1)+(y+1)*L. Here, (ns−1−y) corresponds to adistance between the current predicted pixel 2011 and the second virtualpixel 2014 and (y+1) corresponds to a distance between the currentpredicted pixel 2011 and the adjacent upper pixel 2015. As such, theintra predictors 410 and 550 generate the second prediction value p2through linear interpolation using the distance between the secondvirtual pixel 2014 and the current predicted pixel 2011, the distancebetween the current predicted pixel 2011 and the adjacent upper pixel2015 on the same column as the current predicted pixel 2011, the pixelvalue of the second virtual pixel 2014, and the pixel value of theadjacent upper pixel 2015.

As such, when the first prediction value p1 (x,y) and the secondprediction value p2 (x,y) are obtained via the linear interpolation inhorizontal and vertical directions, the intra predictors 410 and 550obtains the prediction value p(x,y) of the current predicted pixel 2011by using an average value of the first prediction value p1 (x,y) and thesecond prediction value p2 (x,y). In detail, the intra predictors 410and 550 may obtain the prediction value p(x,y) of the current predictedpixel 2011 by using an equation p(x,y)={p1(x,y)+p2(x,y)+nS}>>(k+1),wherein k is log₂ nS.

Alternatively, the intra predictors 410 and 550 may obtain a firstvirtual pixel and a second virtual pixel by using a filtered adjacentupper right pixel and a filtered adjacent lower left pixel instead ofusing an adjacent upper right pixel and an adjacent lower left pixel asthey are.

FIG. 21 is a diagram showing adjacent pixels 2110 and 2120 that arefiltered around a current prediction unit 2100, according to anexemplary embodiment.

Referring to FIG. 21, the intra predictors 410 and 550 generate filteredadjacent pixels by performing filtering at least once on the X adjacentpixels 2110 above the current prediction unit 2100 that is currentlyintra predicted and Y adjacent pixels 2120 to the left of the currentprediction unit 2100. Here, when a size of the current prediction unit2100 is nS×nS, X may be 2 nS and Y may be 2 nS.

When ContextOrg[n] denotes X+Y original adjacent pixels above and leftof the current prediction unit 2100 having the size of nS×nS, wherein nis an integer from 0 to X+Y−1, n is 0 in an adjacent lowest pixel fromamong the adjacent left pixels, i.e., ContextOrg[0] and n is X+Y−1 in anadjacent rightmost pixel from among the adjacent upper pixels, i.e.,ContextOrg[X+Y−1].

FIG. 22 is a reference diagram for describing a filtering process of anadjacent pixel.

Referring to FIG. 22, when ContextOrg[n] denotes original adjacentpixels above and left of a current prediction unit, wherein n is aninteger from 0 to 4 nS−1, the original adjacent pixels may be filteredvia a weighted average value between the original adjacent pixels. WhenContextFiltered1[n] denotes a one-time filtered adjacent pixel, adjacentpixels filtered by applying a 3-tap filter to the original adjacentpixels ContextOrg[n] may be obtained according to an equationContextFiltered1[n]=(ContextOrg[n−1]+2*ContextOrg[n]+ContextOrg[n+1])/4.Similarly, a two-time filtered adjacent pixel ContextFiltered2[n] may begenerated by again calculating a weighted average value between theone-time filtered adjacent pixels ContextFiltered1[n]. For example,adjacent pixels filtered by applying a 3-tap filter to the filteredadjacent pixels ContextFiltered1[n] may be generated according to anequation ContextFiltered2[n],(ContextFiltered1[n−1]+2*ContextFiltered1[n]+ContextFiltered1[n+1])/4.

Alternatively, adjacent pixels may be filtered by using any one ofvarious methods, and then as described above, the intra predictors 410and 550 may obtain a first virtual pixel from at least one adjacentfiltered upper right pixel, obtain a second virtual pixel from at leastone adjacent filtered lower left pixel, and then generate a predictionvalue of a current pixel via linear interpolation as described above.Use of adjacent filtered pixels may be determined based on a size of acurrent prediction unit. For example, the adjacent filtered pixels maybe used only when the size of the current prediction unit is equal to orlarger than 16×16.

FIG. 23 is a flowchart illustrating an intra prediction method accordingto a planar mode, according to an exemplary embodiment.

In operation 2310, the intra predictors 410 and 550 obtain a firstvirtual pixel located on the same line as a current predicted pixel of acurrent prediction unit and corresponding to a pixel located farthestright of the current prediction pixel, by using at least one adjacentpixel located upper right of the current prediction unit. As describedabove, a number of adjacent pixels used to obtain the first virtualpixel may be pre-determined or determined based on a size of the currentprediction unit.

In operation 2320, the intra predictors 410 and 550 obtain a secondvirtual pixel located on the same column as the current predicted pixeland corresponding to a pixel located farthest below the currentprediction unit by using at least one adjacent pixel located lower leftto the current prediction unit. As described above, a number of adjacentpixels used to obtain the second virtual pixel may be pre-determined ordetermined based on the size of the current prediction unit.

In operation 2330, the intra predictors 410 and 550 obtain a firstprediction value of the current predicted pixel via linear interpolationusing the first virtual pixel and an adjacent left pixel located on thesame line as the current predicted pixel. As described above, when alocation of the current predicted pixel is (x,y) wherein x and y is eachfrom 0 to nS−1, an adjacent pixel of the current prediction unit isrec(x,y) wherein x and y is each from −1 to 2*nS−1, a pixel value of anadjacent left pixel is rec(−1,y), a pixel value of the first virtualpixel located at (nS−1,y) is T wherein T is a real number, and aprediction value of the current predicted pixel is p(x,y) wherein x andy is each from 0 to nS−1, the first prediction value p1 (x,y) may beobtained according to an equation p1 (x,y), (nS−1−x)*rec(−1,y)+(x+1)*T.

In operation 2340, the intra predictors 410 and 550 obtain a secondprediction value of the current predicted pixel via linear interpolationusing the second virtual pixel and an adjacent upper pixel located onthe same column as the current predicted pixel. When a pixel value ofthe adjacent upper pixel is rec(x,−1) and a pixel value of the secondvirtual pixel located at (x,nS−1) is L wherein L is a real number, thesecond prediction value p2 (x,y) may be obtained according to anequation p2 (x,y)=(nS−1−y)*rec(x,−1)+(y+1)*L.

In operation 2350, the intra predictors 410 and 550 obtain a predictionvalue of the current predicted pixel by using the first and secondprediction values. As described above, when the first and secondprediction values p1 (x,y) and p2 (x, y) are obtained via the linearinterpolation in horizontal and vertical directions, the intrapredictors 410 and 550 obtain the prediction value p(x,y) of the currentpredicted pixel by using an average value of the first and secondprediction values p1 (x,y) and p2 (x,y). In detail, the intra predictors410 and 550 may obtain the prediction value p(x,y) according to anequation P(x,y)={p1(x,y)+p2(x,y)+nS}>>(k+1) wherein k is log₂ nS).

According to one or more exemplary embodiments, encoding efficiency ofan image can be improved by applying an optimum intra prediction methodaccording to image characteristics via various intra prediction methodsusing adjacent pixels.

One or more exemplary embodiments may be written as computer programsand may be implemented in general-use digital computers that execute theprograms by using a computer readable recording medium. Examples of thecomputer readable recording medium include magnetic storage media (e.g.,ROM, floppy disks, hard disks, etc.), optical recording media (e.g.,CD-ROMs, or DVDs), and storage media. Moreover, one or more of theabove-described elements can include a processor or microprocessorexecuting a computer program stored in a computer-readable medium

While exemplary embodiments have been particularly shown and describedabove, it will be understood by those of ordinary skill in the art thatvarious changes in form and details may be made therein withoutdeparting from the spirit and scope of the inventive concept as definedby the appended claims. Exemplary embodiments should be considered in adescriptive sense only and not for purposes of limitation. Therefore,the scope of the invention is defined not by the detailed description ofexemplary embodiments but by the appended claims, and all differenceswithin the scope will be construed as being included in the presentinvention.

What is claimed is:
 1. An apparatus for intra predicting an image,comprising: a prediction mode determiner configured to acquireprediction mode information indicating one selected from a groupcomprising an inter mode and an intra mode, and intra prediction modeinformation indicating one selected from a group comprising directionalprediction modes and a planar mode from a bitstream, and determine aprediction mode of a current block according to the prediction modeinformation and the intra prediction mode information; and a predictorconfigured to acquire reference samples including a first corner sample,a second corner sample, a first side sample, and a second side sample,the reference samples used for prediction of a current sample, anddetermine a prediction value of the current sample based on a weightedsum of sample values of the first corner sample, the second cornersample, the first side sample, and the second side sample, if theprediction mode of the current block is determined to be the planarmode, wherein, the first corner sample is located at an intersection ofa row to an upper side of the current block and a column to a right sideof the current block, the second corner sample is located at anintersection of a row to a lower side of the current block and a columnto a left side of the current block, the first side sample is located atan intersection of a row in which the current sample is located and acolumn to the left side of the current block, and the second side sampleis located at an intersection of the row to the upper side of thecurrent block and a column in which the current sample is located,wherein, a weight for the first corner sample is determined based on ahorizontal distance between the current sample and the first sidesample, a weight for the second corner sample is determined based on avertical distance between the current sample and the second side sample,a weight for the first side sample is determined based on a distanceless than a horizontal distance between the current sample and the firstcorner sample, a weight for the second side sample is determined basedon a distance less than a vertical distance between the current sampleand the second corner sample.
 2. An encoding apparatus for intrapredicting an image, the encoding apparatus comprising: an encodinginformation determiner configured to: determine prediction modeinformation indicating one selected from a group comprising an intermode and an intra mode, and intra prediction mode information indicatingone selected from a group comprising directional prediction modes and aplanar mode, acquire reference samples including a first corner sample,a second corner sample, a first side sample, and a second side sample,the reference samples used for prediction of a current sample, determinea prediction value of the current sample to be a weighted sum of samplevalues of the first corner sample, the second corner sample, the firstside sample, and the second side sample, if a prediction mode of acurrent block is determined to be the planar mode, determine residualdata indicating a difference between an original value and theprediction value of the current sample; and an output unit configured tooutput a bitstream including the prediction mode information, the intraprediction mode information and the residual data, the first cornersample is located at an intersection of a row to an upper side of thecurrent block and a column to a right side of the current block, thesecond corner sample is located at an intersection of a row to a lowerside of the current block and a column to a left side of the currentblock, the first side sample is located at an intersection of a row inwhich the current sample is located and the column to the left side ofthe current block, the second side sample is located at an intersectionof the row to the upper side of the current block and a column in whichthe current sample is located, weights for the weighted sum aredetermined based on a relative location of the current sample to thecurrent block, a weight for the first corner sample is determined basedon a horizontal distance between the current sample and the first sidesample, a weight for the second corner sample is determined based on avertical distance between the current sample and the second side sample,a weight for the first side sample is determined based on a distanceless than a horizontal distance between the current sample and the firstcorner sample, a weight for the second side sample is determined basedon a distance less than a vertical distance between the current sampleand the second corner sample.
 3. A non-transitory computer-readablerecording medium having embodied thereon computer-readable codes, whichwhen executed by a processor of an encoder causes the encoder to executea method of encoding an image, the method comprising: generating abitstream comprising: prediction mode information indicating oneselected from a group comprising an inter mode and an intra mode for acurrent block; intra prediction mode information indicating one selectedfrom a group comprising directional prediction modes and a planar modefor the current block; and residual data indicating a difference betweenan original value and a prediction value of a current sample in thecurrent block, wherein, when the prediction mode information indicatesthe intra mode and the intra prediction mode information indicates theplanar mode, the prediction value of the current sample is determinedbased on a weighted sum of a first corner sample, a second cornersample, a first side sample, and a second side sample, when theprediction mode information indicates the intra mode and the intraprediction mode information indicates a directional prediction mode, theprediction value of the current sample is determined based on areference sample indicated by a direction of the directional predictionmode, the first corner sample is located at an intersection of a row toan upper side of the current block and a column to a right side of thecurrent block, the second corner sample is located at an intersection ofa row to a lower side of the current block and a column to a left sideof the current block, the first side sample is located at anintersection of a row in which the current sample is located and thecolumn to the left side of the current block, the second side sample islocated at an intersection of the row to the upper side of the currentblock and a column in which the current sample is located, the currentblock is predicted by determining prediction values for samples includedin the current block, weights for the weighted sum are determined basedon a relative location of the current sample to the current block, aweight for the first corner sample is determined based on a horizontaldistance between the current sample and the first side sample, a weightfor the second corner sample is determined based on a vertical distancebetween the current sample and the second side sample, a weight for thefirst side sample is determined based on a distance less than ahorizontal distance between the current sample and the first cornersample, a weight for the second side sample is determined based on adistance less than a vertical distance between the current sample andthe second corner sample.